Project Summary The implementation of vaccines that can specifically boost cell mediated immunity to a pathogen has remained a major challenge in the field and is relevant to multiple protozoan parasites. However, in experimental settings the use of replication deficient live microbes provide one way to generate potent pathogen specific CD4+ and CD8+ T cells responses but many questions remain about the rules that allow the immune system to recognize and process the antigens from these attenuated organisms. For example, even a single low dose of the non-replicating vaccine CPS strain of Toxoplasma gondii elicits a robust parasite specific CD4+ and CD8+ T cell response that provides long-lived memory. This response is dependent on the BATF3-dependent dendritic cells (BATF3 DCs) involved in cross presentation and IL-12 production but our studies indicate that these BATF3 DCs do not directly interact with live parasites, but that directly infected cells are required for T cell priming. These observations lead us to hypothesize that there is a pathway in which infected cells undergoing apoptosis represent the source of parasite antigen for the BATF3 DCs. Therefore, we will use a variety of unique microbial tools (fluorescent reporters, parasite auxotrophs, Cre-exressing parasites) and mouse strains (BATF3 KO, unique reporters, and mice which express inducible forms of the diptheria toxin alpha subunit) combined with our imaging expertise to address I. If BATF3 DC are required for processing and presentation of CPS antigens and II. Whether interfering with the death of the infected cells can be used to modify the CPS- induced responses.